One essentially differentiates between machine tools which operate in the indexing method and machine tools which operate continuously. In the indexing method, a tooth gap is machined, then a relative displacement movement, to move the tool out of a tooth gap, and a so-called single indexing movement (indexing rotation), in which the gear wheel rotates in relation to the tool before the next tooth gap is then machined, occur. A gear wheel is thus manufactured step-by-step. A gear-cutting machine which operates in the indexing method is typically provided with indexing apparatus which rotates the workpiece by one or more indices around the workpiece axis before the tool engages again.
In modern machines, a CNC controller is employed, which is designed in such a way that the indexing movement may be executed at the suitable moment.
The continuous method, sometimes also referred to as the continuous indexing method, is based on more complex movement sequences, in which the tool and the workpiece to be machined execute a continuous indexing movement in relation to one another. The indexing movement results from the coordinated driving of multiple axial drives.
The indexing method has the disadvantage that so-called indexing errors occur. These are caused because the temperature of the workpiece changes during the gear-cutting machining by milling of a workpiece. With increasing temperature, deviations from the presets thus result. Indexing errors also result during the grinding, the errors not occurring due to heating (grinding oil is used in operation), but rather by tool wear during the machining of the individual gaps. The grinding disk is typically dressed again before each new workpiece, so that a similar wear occurs for each workpiece over the individual gaps.
Up to this point, such indexing errors have been compensated for in that the indexing error sum is ascertained and then converted into a compensation. The indexing error sum is typically divided by the tooth count, which results in a so-called linear compensation. This type of compensation is not satisfactory, however, because all teeth are changed in the event of a linear compensation, which may have the result that teeth are changed which were actually seated at the correct location.
Therefore, the invention is based on the object of providing an approach which allows the indexing method in the mass production of bevel gears to be made more precise and to be automated.
The object is achieved according to the invention by a device having a workpiece spindle for receiving a bevel gear, a tool spindle for receiving a tool and multiple drives (X, Y, Z, B, C, A1) for machining the bevel gear in the single-indexing method. The device comprises an interface and is connectable to a measurement system via this interface, the interface being designed in such a way that the device may receive correction values or correction factors from the measurement system in a form to be able to adapt master data or neutral data originally present in a memory of the device. The data is modified on the basis of these correction values or correction factors, before manufacturing of one or more bevel gears on the device is initiated.
This object is achieved according to the invention in that a device is used which is equipped with a workpiece spindle for receiving a bevel gear, a tool spindle for receiving a milling tool, and multiple drives for machining the bevel gear in the single-indexing method. In this single-indexing method, one tooth gap of the gear wheel is machined, then a relative movement is executed between tool and workpiece to remove the tool from the tooth gap, then the bevel gear executes a partial rotation and the milling tool is infed to machine a further tooth gap. According to the invention, the drives are activatable via a controller in such a way that the relative movements and the partial rotations occur so that the indexing error which was ascertained on a prior sample workpiece manufactured on the machine is compensated for in the bevel gear currently to be manufactured in the machine.
This object is also achieved according to the invention in that a special 6-axis device is used for machining a bevel gear, which comprises a workpiece spindle for receiving the bevel gear, a tool spindle for receiving a tool, and drives for machining the bevel gear using the tool. The device executes the following steps of a completing method in which both tooth flanks of a tooth gap are manufactured simultaneously in each case:                predefining master or neutral data which describe the shape of a bevel gear to be mass produced and the machine tool kinematics required for this purpose,        executing the following machining steps in the single-indexing completing method on the basis of the master or neutral data,                    a) machining one tooth gap of a sample workpiece using the tool by executing a machining movement,            b) executing a relative movement between the tool and sample workpiece to remove the tool from the tooth gap,            c) executing an indexing rotation to transfer the sample workpiece into another angular position,            d) machining a further tooth gap of the sample workpiece using the tool by repeated execution of steps a)-c), these steps being repeated until all tooth gaps of the sample workpiece are manufactured,                        ascertaining the indexing error (for example, in a gear-cutting measurement center) of all teeth of the sample workpiece,        ascertaining a suitable indexing error compensation per tooth,        transmitting or providing correction values (offset for the indexing angle and/or the plunging depth of the tool),        adapting the machine data of the 6-axis device on the basis of the correction values as a preparation for the mass production of a series of bevel gears compensated for indexing errors,        production of the bevel gears compensated for indexing errors using the adapted machine data by executing steps a)-d), these steps being repeated until all tooth gaps of a bevel gear compensated for indexing errors are manufactured.        
According to the invention, the control data or machine data are altered by ascertaining the indexing error compensation in such a way that a plurality of the machining movements and the indexing rotations is altered in relation to the original presets which were set during manufacturing of the sample workpiece defined by the master or neutral data.
In other words, the indexing errors are compensated over at least two of the six axes or even over all axes. Thereby at least the rotation is altered by adaptation of the partial rotations and the depth of the tooth gaps is altered by adapting the machining movements, and tooth-to-tooth. The adaptation is not a linear adaptation, but rather an individual adaptation occurs per tooth or per tooth gap, respectively, according to the invention.
I.e., according to the invention each tooth or each tooth gap of the bevel gears to be manufactured in mass production is corrected individually per se, so that each tooth or each tooth gap is seated at the “correct” point. Reference is made to one of the z teeth of the bevel gear. This one tooth is used as a quasi-reference tooth for the compensation of the indexing errors.
The invention is concerned in particular with the dry milling of bevel gears in the single-indexing completing method. The invention is especially suitable for dry milling, because the indexing errors are clearer in dry milling. This is because, inter alia, the temperature is increased more strongly during the milling machining than in the case of wet milling and the machine thus cuts more deeply than “desired”. If the material becomes hotter, the tooth gap typically also becomes larger. Depending on the manufacturing method, the temperature of the workpiece moves from room temperature at the beginning to temperatures between approximately 40 and 50° toward the end of the machining.
The method is also suitable for indexing error compensation in the grinding of gear wheels. During grinding, the grinding disc is dressed before the machining of the component. During the grinding machining, the grinding disc wears away in its height and width, so that the tooth gaps become ever shallower and narrower. The grinding disc is dressed again before the machining of the next component. The compensation method may also be applied in this case.
After ascertaining the indexing error on the sample workpiece, it is ascertained by computer how the indexing angle τ (indexing rotation) and/or the plunging depth (machining movement) must be altered so that the deep cutting or the too shallow cutting may be compensated for in mass production, as described.